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1.
A wife's husband's "other woman" known to the wife; whether openly acknowledged or not.

The legal wife of a woman's, generally a mistress, long-term boyfriend/lover/mate.

Two or more women who share a romantic relationship with one man, but do not share a dwelling/residence.
After spending 25 years with me and simultaneously spending 18 years with her, she was more like a step-sister-wife than my husband's mistress.

Since Warren Buffett never divorced his wife Susie, she and his long-term partner, Astrid, are, for all intents and purposes, step-sister-wives.
by AmberChella March 01, 2011